Congestive heart failure (CHF) is a leading cause of hospitalization and death in the United States, and its incidence is increasing. Secondary mitral regurgitation (MR), a complication of end-stage cardiomyopathy, refers to the backflow of blood from the left ventricle to the left atrium resulting from imperfections in the mitral valve. When the mitral valve allows blood to flow backward into the left atrium, the left ventricle must pump progressively harder to circulate blood throughout the body, which in turn promotes CHF. While heart transplantation is considered a standard treatment for select patients with severe CHF and end-stage heart disease, it is only applicable to a small percentage of patients because of the small number of available donor hearts and surgical risks for weaker patients. Accordingly, alternative medical and surgical strategies are evolving to treat such conditions.
As seen in FIGS. 1A and 1B, the mitral aimulus 20 represents the junction of the fibrous and muscular tissue that joins the left atrium. LA and left ventricle LV. The average human mitral annular cross-sectional area is 5-11 cm2. The mitral valve is a bicuspid valve having a large posterior leaflet 22 that coapts or meets with a smaller anterior leaflet 24. The anterior aspect 26 of the annulus, which is in continuity with the fibrous skeleton of the heart, has limited flexibility, whereas the posterior aspect 28 of the annulus, which is not attached to any rigid surrounding structures, has more flexibility. For the purpose of discussion, the mitral annulus 20 lies generally in a datum plane 30 (FIG. 1A) at an angle with respect to a datum plane 32 in which the aortic valve 34 is generally oriented. These datum planes 30, 32 can be defined as being perpendicular to the average blood flow through the respective valves. During systole the mitral annulus 20 assumes a generally elliptical shape as shown in FIG. 1B, and is able to contract and decrease in diameter, whereas, in diastole, it assumes a more circular shape and opens to permit blood to fill the left ventricle LV. Annular flexibility allows for increased leaflet coaptation during systole and increased annular orifice area during diastole.
In MR, dilation typically occurs along the more flexible posterior aspect 28 of the annulus, as seen in FIGS. 2A and 2B. Some patients experiencing a drop h of the posterior aspect 28 of the mitral valve annulus, as seen in FIG. 2A, and consequent relaxation of the posterior muscle wall 36 of the left ventricle LV. FIG. 2B illustrates the lengthening of the anterior-posterior dimension 38 and subsequent loss of coaptation between the posterior and anterior leaflets 22, 24.
MR leads to a cycle of continuing volume overload of the already dilated left ventricle LV, progression of annular dilation, increased left ventricle wall tension, increasing degrees of MR and worsening CHF. In MR, the regurgitant volume ejected into the left atrium LA is dependent upon mitral orifice size, ventricular/atrial pressure gradient and heart rate. The regurgitant flow into the left atrium LA increases left atrial pressure, which leads to atrial enlargement and an increase in compliance, and decreases forward systemic flow. Left atrial pressures rise during systole and decline in diastole.
FIGS. 3A and 3B illustrate the use of a Carpentier-Edwards PHYSIO annuloplasty ring 40 to restore the original healthy shape of the mitral annulus 20. The ring 40 is typically semi-rigid and planar and restores the primary anterior-posterior dimension 38′ of the mitral annulus 20.
Various other interventions have been used to alter the size of the regurgitant orifice area. An increase in preload or afterload, or a decrease in contractility, results in dilation of the LV and an increase in regurgitant orifice area. The complex relationship between mitral annular area and leaflet coaptation may explain why some studies have found that performing a “valvular” repair, with an undersized flexible annuloplasty ring, has helped with a “muscular” problem of the left ventricle. For example, in a study conducted between 1993-1999 at the University of Michigan, 92 patients with end-stage cardiomyopathy and refractory MR underwent mitral valve repair with an “undersized” annuloplasty rings having a circumference smaller than that of the patient's annulus in its natural, pre-diseased state.
Annuloplasty rings have also been developed in various shapes and configurations over the years in an effort to correct MR and other conditions which reduce the functioning of the valve. For example, Carpentier, et al. in U.S. Pat. No. 4,055,861 disclosed two semi-rigid supports for heart valves, one of which being closed (or D-shaped) and the other being open (or C-shaped). In the closed configuration, the ring is generally flat about an anterior-posterior plane, and has a convex posterior side and a generally straight anterior side. U.S. Pat. Nos. 5,104,407, 5,201,880, and 5,607,471 disclose closed annuloplasty rings that are bowed slightly upward on their anterior side. Because the anterior aspect 26 of the mitral annulus is fibrous and thus relatively inflexible (at least in comparison to the posterior aspect 28), the upward curve in the anterior side of each ring conforms the ring more closely to the anatomical contour of the mitral annulus, and thus reduces undue deformation of the annulus.
It should be noted here that correction of the aortic annulus requires a considerably different ring then with a mitral annulus. For example, U.S. Pat. Nos. 5,258,021 and 6,231,602 disclose sinusoidal or so-called “scalloped” annuloplasty rings that follow the up-and-down shape of the three cusp aortic annulus. Such rings would not be suitable for correcting a bicuspid valve deficiency.
While good results in the treatment of CHF and MR have been obtained in the preliminary applications of the above-described methods and apparatuses, it is believed that these results can be significantly improved. Specifically, it would be desirable to produce a mitral annuloplasty ring that can re-shape the mitral annulus in a way that will significantly repair the geometric configuration of the left ventricle wall beyond that which has been observed with undersized rings.